The present invention relates to a device for coupling between, on the one hand, the delay line of a power travelling wave tube, in which the electron beam is focused by means of alternate permanent annular magnets and, on the other hand, an external circuit for transmitting the energy of the tube, formed by a wave guide with double ridge; the invention relates more especially to a device having a small wave guide, generally with a single ridge, of reduced dimensions so as not to exceed the thickness, measured parallel to the axis of the tube, of that one of the permanent magnets which it passes through for providing coupling between the delay line situated inside the focuser and the double ridge wave guide situated outside the focuser. In the context of this disclosure, the term "ridge" is defined as being parallel to the axis of the waveguide, as opposed to a "step" which is defined as being perpendicular to the axis of the guide.
Such coupling devices are known from the French patent 2 485 801 filed on the 27 June 1980, where the small wave guide is a straight guide which opens into an output wave guide, with one of the two ridges of the output guide forming an extension of the ridge of the small guide and the other of the two ridges which gradually increases until it disappears before reaching the end of the double ridge guide coupled to the small guide. Such a coupling device, an embodiment of which will be described in connection with FIG. 1 of this text, has an insufficient band width for certain applications; in addition, because of its construction, it is fairly bulky.